1. Field of the Invention
The present invention relates to a method for producing aliphatic polyester and a method for using glucans such as starch, cellulose etc. as a resource.
2. Related Background Art
The conventional general-purpose plastic products are composed of polymers synthesized from petroleum resources. More specifically, polymer products such as polyester, polystyrene, nylon, polyethylene, polyvinyl chloride, polyimide, polycarbonate etc. are all synthesized from petroleum. However, the petroleum is a limited resource which is to run out sooner or later. For this reason there is strongly desired a technology for producing the general-purpose plastic products from a new raw material capable of substituting petroleum, namely a recyclable raw material.
On the other hand, starch is a polymer compound formed by dehydration polymerization of D-glucose, and is an important polysaccharide comparable to cellulose. Starch is produced from potato, sweet potato, corn etc., with the worldwide production (production amount of corn) amounting to 400 to 500 million tons per year, and is a recyclable resource having the largest production amount among the natural resources. Starch can highly be expected as a new resource which replaces the petroleum, if general-purpose plastic products can be produced therefrom.
The object of the present invention is to provide a method for producing aliphatic polyester utilizing glucans such as starch or cellulose as a raw material.
Based on a standpoint that a novel technical development is required to provide against the exhaustion of the petroleum resources in the future, the present inventors through intensive investigation have noticed starch as a raw material which can replace petroleum and have found that aliphatic polyester can be synthesized from caproic acid that can be obtained from starch via glucose, thereby attaining the present invention. This finding opens up a way for utilizing starch as an efficient resource in obtaining plastics of high quality from starch as a starting material.
The above-mentioned object can be attained, according to an embodiment of the present invention, by a method for producing an aliphatic polyester represented by the following formula (I): 
(wherein n stands for an integer within a range from 5 to 10,000), the method comprising the steps of:
(i) hydrolyzing starch to obtain glucose;
(ii) oxidizing the glucose to obtain gluconolactone;
(iii) reducing the gluconolactone to obtain caproic acid;
(iv) chlorinating the caproic acid to obtain 6-chlorocaproic acid;
(v) cyclizing the 6-chlorocaproic acid to obtain xcex5-caprolactone represented by the following formula (II): 
and
(vi) executing ring-opening polymerization of the xcex5-caprolactone.
The aforementioned object can be attained also, in another embodiment of the present invention, by a method for producing an aliphatic polyester represented by the following formula (I): 
(wherein n stands for an integer within a range from 5 to 10,000), the method comprising the steps of:
(i) hydrolyzing starch to obtain glucose;
(ii) oxidizing the glucose to obtain gluconic acid;
(iii) reducing the gluconic acid to obtain caproic acid;
(iv) chlorinating the caproic acid to obtain 6-chlorocaproic acid;
(v) cyclizing the 6-chlorocaproic acid to obtain xcex5-caprolactone represented by the following formula 
and
(vi) executing ring-opening polymerization of the xcex5-caprolactone.
xcex5-caprolactone is a compound having an intramolecular cyclic ester structure and is well known as an industrially producible compound by oxidizing cyclohexanone. It is also known that xcex5-caprolactone easily undergoes ring-opening polymerization to provide aliphatic polyester (Japanese Patent Application Laid-Open No. 11-158172). However, there have not been known examples, except that of the present inventors, of synthesizing xcex5-caprolactone from starch and obtaining aliphatic polyester therefrom.
According to the present invention, there is also provided a method for producing an aliphatic polyester represented by the following formula (I): 
(wherein n stands for an integer within a range from 5 to 10,000), the method comprising the steps of:
(i) hydrolyzing glucan to obtain glucose;
(ii) oxidizing the glucose to obtain gluconolactone or gluconic acid;
(iii) reducing the gluconolactone or the gluconic acid to obtain caproic acid;
(iv) chlorinating the caproic acid to obtain 6-chlorocaproic acid;
(v) cyclizing the 6-chlorocaproic acid to obtain xcex5-caprolactone represented by the following formula (II): 
and
(vi) executing ring-opening polymerization of the xcex5-caprolactone.
According to the present invention, there is also provided a method for producing an aliphatic polyester represented by the following formula (VI): 
(wherein n stands for an integer within a range from 10 to 6,000), the method comprising the steps of:
(i) hydrolyzing glucan to obtain glucose;
(ii) oxidizing the glucose to obtain gluconolactone or gluconic acid;
(iii) reducing the gluconolactone or the gluconic acid to obtain caproic acid;
(iv) chlorinating the caproic acid to obtain 5-chlorocaproic acid;
(v) cyclizing the 5-chlorocaproic acid to obtain xcex4-caprolactone; and
(vi) executing ring-opening polymerization of the xcex4-caprolactone.
The method of the present invention for producing aliphatic polyester, in an embodiment thereof, comprises the steps of:
(i) hydrolyzing starch to obtain glucose;
(ii) oxidizing the glucose to obtain gluconolactone or gluconic acid;
(iii) reducing the gluconolactone or the gluconic acid to obtain caproic acid;
(iv) chlorinating the caproic acid to obtain 6-chlorocaproic acid;
(v) cyclizing the 6-chlorocaproic acid to obtain xcex5-caprolactone; and
(vi) executing ring-opening polymerization of the xcex5-caprolactone to obtain aliphatic polyester represented by the foregoing formula (I).
The method for synthesizing aliphatic polyester from starch opens up a novel way of utilizing starch as a resource. From this standpoint, the method of the present invention for producing aliphatic polyester also provides a useful method for utilizing starch as a new resource.
In the following there will be explained each of the aforementioned steps (i) to (vi).
Step (i) (Starch to Glucose)
Conversion from starch to glucose can be achieved for example by hydrolysis with a dilute acid such as sulfuric acid, hydrolysis with an enzyme such as amylase or maltase, or hydrolysis with ultracritical water. The step of obtaining glucose from starch is preferably executed by hydrolysis with an acid. The reaction conditions can be suitably determined according to the already known method.
Step (ii) (Glucose to Gluconolactone or Gluconic Acid)
Conversion from glucose to gluconolactone can be achieved for example by bromine oxidation of glucose or by a method utilizing notatin which is a glucose oxidase. The step of obtaining gluconolactone from glucose is preferably executed by bromine oxidation. The reaction conditions can be suitably determined according to the already known method.
Conversion from glucose to gluconic acid can be achieved for example by oxidation with bromine and concentrated sulfuric acid, more specifically oxidizing and hydrolyzing glucose in sulfuric acid saturated with bromine, or by electrolytic oxidation of a glucose solution or by fermentation of gluconic acid utilizing bacteria of Penicillium family. The step of obtaining gluconic acid from glucose is preferably executed by oxidation utilizing bromine and concentrated sulfuric acid. The reaction conditions can be suitably determined according to the already known method.
Step (iii) (Gluconolactone or Gluconic Acid to Caproic Acid)
Conversion of gluconolactone or gluconic acid to caproic acid can be achieved for example by reduction thereof with hydroiodic acid and red phosphorus. In this reaction, it is desirable that the hydroxyl group alone of gluconolactone or gluconic acid is oxidized.
The amount of red phosphorus employed in the reduction is preferably 1.8 to 2.4 equivalents with respect to gluconolactone or gluconic acid. Hydroiodic acid employed in the reduction preferably has a concentration of 50 to 60 mass %, and is preferably employed in a weight of 40 to 60 times with respect to the weight of gluconolactone or gluconic acid. The reducing reaction is completed by refluxing gluconolactone or gluconic acid and red phosphorus in hydroiodic acid for about 20 hours. The reaction mixture is filtered, then the filtrate is extracted with ether and washed with an aqueous solution of sodium hydrosulfite of about 5 mass %, and caproic acid can be obtained by distilling the ether solvent and executing vacuum distillation.
Step (iv) (Caproic Acid to 6-Chlorocaproic Acid)
Conversion from caproic acid to 6-chlorocaproic acid can be achieved for example by chlorination with chlorine and concentrated sulfuric acid, preferably by chlorination conducted by reacting caproic acid with chlorine in concentrated sulfuric acid. The reaction conditions can be suitably determined according to the known method.
Step (v) (6-Chlorocaproic Acid to xcex5-Caprolactone)
Conversion from 6-chlorocaproic acid to xcex5-caprolactone can be achieved for example by cyclization utilizing an aqueous solution of sodium hydroxide, preferably by boiling 6-chlorocaproic acid in an aqueous solution of sodium hydroxide. The 5 reaction conditions can be suitably determined according to the known method.
Step (vi) (xcex5-Caprolactone to Aliphatic Polyester; Ring-opening Polymerization)
In the present invention, aliphatic polyester is synthesized by ring-opening polymerization of xcex5-caprolactone utilizing a compound having a hydroxyl group as an initiator normally in the presence of a catalyst. The initiator is used for opening the ring of xcex5-caprolactone, and the catalyst accelerates the polymerization by interacting with the ring-opened product.
In the present invention, a known ring-opening polymerization catalyst can be employed as the polymerization catalyst in the ring-opening polymerization of xcex5-caprolactone. Examples of such catalyst include tin dichloride, tin tetrachloride, tetra-n-butoxy-germanium, tetramethoxy-germanium, tetraethoxy-germanium, triethoxy-aluminum, tri-n-propoxy-aluminum, tri-iso-propoxy-aluminum, tri-n-butoxy-aluminum, tri-iso-butoxy-aluminum, aluminum chloride, triethyl-aluminum, trimethyl-aluminum, di-iso-propyl zinc, dimethyl-zinc, diethyl-zinc, zinc chloride, tetra-n-propoxy-titanium, tetra-n-butoxy-titanium, tetra-t-butoxy-titanium, tetraethoxy-zirconium, tetramethoxy-zirconium, tetra-iso-propoxy-zirconium, tetra-n-butoxy-zirconium, tetra-iso-butoxy-zirconium, tetra-t-butoxy-zirconium, and organic compounds of rare earth metals such as La, Nd, Sm, Er, Tm, Yb or Lu. Such catalyst may be employed singly or as a mixture of at least two catalysts.
The amount of polymerization catalyst can be determined suitably, but is usually within a range of 0.01 to 10 wt. %, preferably 0.05 to 5 wt. % with respect to the total amount of e-caprolactone and the polymerization initiator.
In the present invention, a known polymerization initiator can be employed in the ring-opening polymerization of xcex5-caprolactone. Examples of such polymerization initiator include monools such as methanol, ethanol, 1-propanol, 2-propanol, butanols or phenol, diols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol or 1,10-decanediol, triols such as glycerin or trimethylol propane, and polyols such as neopentyl glycol or pentaerythritol. Such initiator may be employed singly or as a mixture of at least two initiators.
The molar ratio of the polymerization initiator to be employed in the present invention and xcex5-caprolactone can be suitably selected according to the polymerization ratio of the desired aliphatic polyester, and is normally within a range of 1:1 to 1:5,000, preferably within a range of 1:1 to 1:2,000.
The ring-opening polymerization of xcex5-caprolactone can be executed by a polymerization reaction of xcex5-caprolactone in the presence of the polymerization catalyst and the polymerization initiator under the presence of inert gas or under a reduced pressure. The ring-opening polymerization of xcex5-caprolactone is preferably executed in a nitrogen atmosphere for the ease of operation.
In the ring-opening polymerization of xcex5-caprolactone, the reaction temperature and time can be arbitrarily selected. The reaction temperature is preferably equal to 50xc2x0 C. or higher, particularly 100xc2x0 C. or higher in order to obtain a sufficiently high reaction speed, and is preferably not exceeding 200xc2x0 C., particularly not exceeding 180xc2x0 C. in order to substantially avoid coloration of aliphatic polyester by oxidation or decomposition of the generated aliphatic polyester. Also the reaction time can be arbitrarily selected within a range not affecting the quality of the generated aliphatic polyester.
The ring-opening polymerization of xcex5-caprolactone can also be executed in a solvent. The solvent is preferably an inactive solvent not reacting with xcex5-caprolactone, polymerization catalyst or polymerization initiator, selected from aromatic hydrocarbons such as toluene or xylene, or aliphatic or alicyclic hydrocarbons such as hexane or cyclohexane. Preferably such solvent is substantially anhydrous.
The weight-average molecular weight of aliphatic polyester obtained by the ring-opening polymerization of xcex5-caprolactone is preferably 1,000 or higher, particularly 30,000 or higher in terms of polystyrene and preferably 1,000,000 or lower, particularly 500,000 or lower in terms of polystyrene.
The aliphatic polyester of the present invention thus obtained can be utilized in various industrial fields by modifying the weight-average molecular weight or the functional group contained therein. For example, the aliphatic polyester of a weight-average molecular weight of 1,000 to 5,000 utilizing glycol as a polymerization initiator is extremely useful, exploiting the presence of a hydroxyl group therein, as a raw material for polyurethane or paints. Also the aliphatic polyester having a weight-average molecular weight exceeding 50,000 has a practical mechanical strength and is usable in plastic molded articles, films or hot-melt adhesives. The molding can be executed, for example, by compression molding, injection molding, extrusion molding, mold casting or transfer molding utilizing a mold.
Also the aliphatic polyester of the present invention may be mixed, within a range not affecting the object of the present invention, with another resinous component, a rubber component, a heat resistance stabilizer, a flame retarding agent, a slipping agent, an antiblocking agent, an anticlouding agent, a friction reducing agent, a filler, a dye, a pigment, natural oil, synthetic oil or wax. The mixing ratio is not particularly limited and can be suitably determined.
In the following, the present invention will be further clarified by way of examples, but the present invention is by no means limited to such examples.